Can you get more concrete on why you would want to mix formats? So far what I have seen namespaces were used just to extend one format.

> Now let's say both the RSS-feed and the custom-data contains both a "link" element and a "title" element.

Easily avoided if you just extend one format, but can happen if you would want to mix existing formats because of some reason.

> - How can a RSS-library validate that the feed format itself is correct?

You still need some code that parses the non-RSS part anyway, which can validate it. Don't you usually have just one parser that parses RSS and the non-RSS at the same time? If not, how are you passing your non-RSS XML tree-parts from the RSS-only library to the "new stuff"?